Carbon nanotubes (CNTs) were first synthesized and reported by Prof. S. Iijima of NEC Corporation, Japan in Nature 354, 56-58, 1991, and the polymer nanocomposites using carbon nanotube as filler were first reported by Prof. P. Ajayan et al. in Science, 265, 1212-1214, 1994. One graphene layer folds along the axis produces single walled carbon nanotubes (SWCNT) whereas many graphene layers wrapped onto themselves makes multi walled carbon nanotube (MWCNT). Carbon nanotubes (CNTs) are a novel crystalline form of carbon and the scientific community across the world soon realized experimentally and theoretically the CNT's unique atomic structure and properties, such as, high flexibility, low mass density, high aspect ratio, high strength-to-weight ratio, and extraordinary electrical, thermal, mechanical properties. J. P. Lu, in Physical Review Letters, 79, 1297-1300, 1997 and E. W. Wong et al in Science, 277, 1971-1975, 1997 has reported that the axial elastic modules and tensile strength of SWCNTs are theoretically and experimentally predicted to be as high as 1-2 TPa and 200 GPa respectively. Although the physical and chemical properties of SWCNT's are much superior to MWCNTs, however MWCNTs are widely used for application purpose due to their relatively low production cost and availability in large quantity.
E. T. Thostensona et al in Composites Science and Technology, 61, 1899-1912, 2001. has reported that the large surface area, high modules and strength of CNTs make them a good candidate for reinforcing host matrixes like polymer, ceramic or metal. Recent experimentations have shown remarkable enhancements in mechanical strength of composite with an addition of small amounts of CNTs, however, there are several challenges that are still need to be overcome in order to achieve the full potentials of CNT based composites as reported by R. Andrews et al, in Current Opinion in Solid State and Materials Science, 8, 31-37, 2004. The three critical issues of the CNT based composites are the uniform dispersion of CNTs in the host matrix material, the second is the proper interaction between the CNT and the host matrix and the third is the alignment of CNTs within the matrix. To achieve the distribution of CNTs in the matrix, the modification of the surface of CNTs is required either by covalent or noncovalent functionalization. For the proper interactions between the CNT and the host matrix the judicious choice of functional group on the CNT surface is critical.
S. Banerjee et al in Advance Materials, 17, 17-29, 2005 has reported that the chemical functionalization of CNTs allows the surface modification of carbon nanotube by introducing different functional groups for the better dispersion in organic solvent. Generally, CNTs are chemically modified either by covalently attach the functional group to the CNT surface or by wrapping polar/nonpolar molecules on the surface of the CNT by noncovalent interactions. Covalent functionalization of CNTs is very effective to enhance the proper dispersion of CNTs in the matrix, however, the covalent bonding inevitably disrupt the long range π conjugation along the CNT axis, leading to the defects on the CNT side walls. Covalent functionalization seriously affects the electrical properties as well as mechanical properties of CNT. Consequently by keeping the CNTs structure intact, the use of noncovalent interactions such as π-π interactions, van der Waals interactions and static charge interaction have been attempted to wrap the different molecules and polymers on CNTs surface for the proper distribution in the host matrix. However, it is realized that not only the distribution, the interaction of the attached moieties over CNTs with the host matrix is utmost important for achieving higher mechanical strengths. Therefore, the choice of functional group in both covalent and noncovalent derivatives' of MWCNTs is critical to achieve high reinforcement effect.
Jifen Wang et al in Journal of material Science technology, 2011, 27 (3), 233-238 disclosed the preparation of oleylamine derivative or oleylamine grafted of MWCNTs by process that involves: treating MWCNTs with nitric and sulphuric acids followed by a treatment with SOCl2 (containing dimethylformamide (DMF)) to covert the —COOH groups to —COCl and treating the resulting sample with oleylamine to obtain oleylamine derivative of MWCNTs. The oleylamine groups in the functionalized multi walled carbon nanotube (MWCNT) prepared by this process are mostly non-covalently coated on MWCNTs. Further, the acids used in this process are concentrated acids that damage the structure of MWCNTs. The oleylamine coating on MWCNTs prepared by this process would easily come out during sonication in organic solvents and thus the applications of these MWCNTs are restricted.
There are several covalent derivatives of MWCNTs schemes available in the literature, which produced functionalized nanotubes that can be distributed in the polymer/resin matrixes. However, the enhancement of mechanical strength of the composite is not always showed desired results due to the improper reinforcement.
Thus there is a need for a process by which the functionalized MWCNTs can be synthesized in which the functional group is covalently attached to MWCNTs surface.